The one that, on page one, says, "Do it."
and on page two:
"Good, again!"

seriously though, "best" is subjective. Some folks want a lot of pictures and big print with small words, others like to hear it from the horse's mouth and only buy books (ghost) written by big name athletes, others want the technical research laid out for them to decide for themselves. Then there're those who want inspiring stories, but who can't be told how to do anything because they know what works for them, or who want Chris Lieto to tell them exactly what he did to break the IMC bikesplit so they can do that too, as if what works for Chris will work for some girl, or a guy who weighs 50lbs more than he does.

What exactly are you looking for in a "training book?"

Geoff

Lots of good choices at http://www.velopress.com/triathlon.php

I recommend my book, The Triathlete's Guide to Run Training, available at Fitness-Concepts.com Below is more information about it. Ken

Written by Exercise Physiologist Ken Mierke, coach to many of triathlon’s fastest runners, this book covers every aspect of preparing to run fast after hard cycling. Topics include reaching race weight, efficient technique, cycling techniques that enable fast running, psychological mode for optimal performance, and many others. Read the most comprehensive guide to triathlon running ever written.